Method of making hobs



J. W. BRUSSEL 1334 558 METHOD OF MAKING HOBS Original Filed June 1927 2Sheets-Sheet l ht W Nov. 5, 192%. J. w. BRUSSEL 1,734,558

METHOD OF MAKING HOBS Original Filed June 30, 192 2 Sheets-Sheet 2Patented Nov. 5, 1929 UNITED STATES PATENT OFFICE JOHN W. BRUSSEL, 0FDETROIT, MICHIGAN, ASSIGNOR TO THE TIMKEN-DETROIT AXLE COMPANY, OFDETROIT, MICHIGAN, A CORPORATION OF OHIO METHOD OF MAKING HOBS Originalapplication filed June 80, 1927, Serial No. 202,486. Divided and thisapplication filed August 23,

1928. Serial My invention relates to a method of making hobs or helicalcutters, particularly hobs of the type that are adapted to generate wormwheels by the tangential feed method. Its principal objects are todevise a method of making a new hob which constitutes the subject matterof my copending application Serial No. 202,486, filed June 30, 1927, nowPatent No. 1,728,198, and of which this application is a division. Theinvention consists in the method hereinafter described and claimed.

In the accompanying drawing wherein like reference letters and numeralsrefer to like parts wherever they occur:

Fig. 1 shows the front elevation of my improved hob in which theroughing, finishing. and burnishing portions are indicated.

by the letters A, B, and C, respectively;

Fig. 2 is a diagrammatic view showing the development of the new hobteeth in the pitch plane;

Fig. 3 is a diagrammatic view showing the (lQYOlOl'HIlCIlt of a hobthread shown in Fig. 2 in elevation;

Fig. 4 is a diagrammatic View of a plane section of the new hob takenthrough the hob axis- Fig. 5 is a section perpendicular to the axis;

Fig. 6 is a detail view of two consecutive hob teeth showingprogressively increasing cylindrical lands; and

Figs. 7 and 8 are geometrical diagrams, explanatory of the calculationof the new hob helixes.

The principle of worm gear bobbing by means of the tangential feed iswell known and consists in constructing a worm hob that has the samepitch diameter, helix angle, and form of tooth as the worm which itrepresents, in providing the entering end of said hob with a taperedportion followed by a cylindrical portion, in setting said hob in abobbing machine at the predetermined center distance from the blank tobe cut, in rotating the two members in a fixed relation, and inimparting to the hob a translatory or tangential feed in the directionof its axis. By this means all the teeth in the blank are finished onboth their sides and ina single cutting operation. Heretofore, the teethof a hob have been formed by intersecting the hob threads with aplurality of helical flutes or gashes, said flutes being of an oppositehand to that of the hob threads and so selected that, first, theyintersect the hob threads at preferably right angles and, second,produce radial cutting faces in hob teeth, i. e., faces perpendicular tothe hob axis,- the remaining truncated portions of the hob threads beingrelieved in a relieving lathe so that the cutting faces will stand outprominently relative to the rear portions of the teeth and thereby willpossess a cutting clearance. In consequence .of such relieving orbacking off of the teeth to produce a cutting clearance, such hobs wereof comparatively short life, because the sharpening thereof would reducethe diameter of the hob and would change the form of the teeth and thehelical angle, all of which tended to produce inaccurate gears.

I have discovered that when the tangential feed principle of worm geargenerating is employed, it is not necessary to provide all cutting teethwith the same degree of relief or cutting clearance, but that saidclearance may be distributed from the front or entering end of the hobtoward its rear end in an ever diminishing ratio without affecting thefree cutting'qualitiesof said hob. Accordingly, in my preferredconstruction, I divide the body of the preferably integral hob intothree portions, viz, a roughing portion A, a finish-cutting portion B,and a burnishing portion C, the cutting clearance of each successivetooth of the roughing and finishcutting portions being less than that ofthe preceding tooth, and the teeth of the burnishing portion havingpractically no cutting clearance. The entering or roughing portion A istapered and its teeth do the initial and rough cutting; the middleportion B is cylindrical and its teeth do the finish'-cutting; and thefinal portion C is cylindrical and its teeth burnishthe work cut by thepreceding teeth.

My new hob is laid out as follows: First a straight line h 'is laid outrepresenting the pitch helix of the hob teeth, which line has a helicalangle d relative to the plane 12 which is perpendicular to the axis 13of the hob. A straight line is then laid off at right angles to thehelix line h of a length equal to the width of the first tooth of thefinishing section of the hob, the ends M and N of this line beingequidistant from the helix line h Through the points M and Nrespectively, are drawn two lines 72, and h" parallel to the pitch helixline h and representing the primary helixes of the new hob. Also throughsaid points M and N are drawn straight lines h, and it, that convergetoward the helix line h, and make an angle a therewith. These lines 72.and

h represent the secondary helixes and have respectively the helix angle(l-l-a and 'd-a. The teeth are then laid off at equi-distant interavlsalong the helix line h with their cutting faces represented by lines atright angles thereto and extending from one secondary helix line It, tothe other secondary helix line k By this arrangement, the foremost toothis the narrowest, and the width of each successive tooth increases up toand including the first finishing tooth M and N whose ends lie at thepoints of intersection of the primary and secondary helixes. At thispoint, of course, the distance between the secondary helixes is the sameas the distance between the primary helixes.

The width of all of the finishing and burnishing teeth is the same,namely the distance between the primary helix lines h and it. However,although the faces of the finishing or burnishing teeth are of equalwidth, they are of diiferent form, as will appear from Fig. 2 of thedrawing. In laying off the finishing or burnishing teeth their facelines are extended to intersect with the lines h and h, representing thesecondary helixes and, from these points of intersection, lines parallelwith the lines representing the re lieved sides of the roughing teethare drawn, which side lines intersect the lines h and k representing theprimary helixes. These points of intersection with the primary helixlines are further and further from the face line for each successivefinishing and burnishing tooth. In this layout, all of the roughingteeth and the first finishing tooth are represented by trapeziums ofequal height but different widths, whereas all teeth subsequent to thefirst finishing tooth can be considered as continuing the series oftrapeziums but with their face corners cut awayalong the primary helixlines h and h.

In practice the hob is made as'follows: The hob teeth are first milledparallel to the helix h leaving sufilcient stock for the subsequentoperations.. Then the tops of the teeth are relieved or backed-off alongthe cone C and cylinder C, Fig. 3, with the desired amount of topclearance 12. Then the relieving machine is geared up to a lead 1 ofhelix shorter than the lead 1 of the pitch helix h Fig. 2, and thecutting edges- M, N,of the first entering tooth T progressively towardthe rear end of the hob.

After the hob thus partially machined has been hardened, the formationof the improved cutting teeth is completed by trimming said teethcylindrically (without giving the cutting teeth any further relief)preferably by means of a grinder along the helixes h and it" Fig. 2, thelead of helix being now 1. The effect of this operation will be that theroughing teeth M to M inclusive will remain untouched, while the tooth Twill just barely be touched at the points M N the latter distancecorresponding exactly to the predetermined width of tooth necessary togenerate correct worm wheels with the required amount of back lash. Thenext cutting tooth T will be trimmed off to P", Q], both of said widthsbeing exactly equal to the finishing width M N of the first finishingtooth T It is seen that in this manner a series of similar triangles MP", S, M, P, S are trimmed ofi from the consecutive teeth T T etc, saidtriangles being all geometrically similar, but of an ever increasingarea. This, in eifect, produces upon the front faces of the burnishingteeth a series of cylindrical lands P,

8 6 P, S=e", said lands 6, e, etc., being of an ever increasing width.

The formation of a series of ever broadening cylindrical lands e, 6,etc., on the front faces of the burnishing teeth T T etc., may also bestudied in Figs. 3, 5 and 6, which diagrams will be readily understoodfrom the preceding explanation. In Fig. 4, the conditions governing theincreasing widths of the improved hob teeth T to T may be observed incomparison with the standard rack R, the latter being shown in dottedlines superposed upon the improved teeth. It will be seen that afterthe-first finishing tooth T has beenreached the widths of su sequentcutting teeth T and T will no longer increase, 'but'will remainconstant.

The practical importance of my invention will .best be understood byexamining Fig. 5. The first finishing tooth T will cut the worm gearteeth to-their full size, theoretically. In practice, however, Iobserve'tliat, due to the vibration of the bobbing machine and othercauses, some little stock may remain uncut by the tooth T which stockwill be removed by the subsequent teeth T, T, etc. As said teeth have aline bearing upon the finished gear teeth, it follows that first theywill lessen the vibration of the hob because they will provide a seriesof additional points of support to the hob and second, said teeth willburnish to ear. tooth surfaces and produce a smoother finish. Thus, ahob of this kind will remain sharp for a longer period of cutting timebecause of the lessened vibration. I

When the hob teeth become dull and are resharpened, Fig. 5, a portionfrom the faces of each tooth T, T, etc., is ground off. Suppose that theamount thus taken off is exactly equal to the width of the land e of thetooth T. vIt is seen then that the tooth T will become the last roughingtooth instead of being the first finishing tooth as formerly, and T willnow become the first finishing tooth. The cylindrical lands 0,-

e 6 etc., will now all be narrower than formerly because the width a hasbeen subtracted from each. Thus, the only difference in the cuttingaction will be that the number of roughing teeth has been increased byone, and the number of finishing teeth has been decreased by one. Ifthere be a sufficiently large number of finishing teeth in this hob, itis evident that the hob may be resharpened a great many times before itloses itssize or helical angle, thus materially reducing the tool cost.

The calculation of the secondary base helixes h, and k will now beexplained. As seen from Figs. 7 and 8, the lead of the primary helix kis equal to 1 and the helix angle of the same is equal to d. Let thepitch radius of the hob be denoted with R, then 1=2Rqr tan d (1) 1 =2 Rtan a+d) 1 =2 R 1r tan ad) Thus the leads of the secondary helixes maybe determined because all the quantities on the right side of theEquations (2) and (3) entering end of the hob to the rear end thereof,in relieving the sides of teeth along two corresponding helixesdiverging from the entering toward the rear end of the hob and incylindrically trimmingofli' the sides of all teeth the width of which isin excess, .of the predetermined width of tooth.

2. A method of manufacturing worm hobs, Y

3. A method of manufacturing worm hobs from an integral screw thread,which consists in forming the teeth in the roughing portion thereof upona truncated cone and radially relieved, in forming the teeth in theadjacent finishing portion upon a cylinder and radially relieved and informing the following burnishing portion upon a cylinder and unrelieved.

4. A method of manufacturing solid worm hobs having three cuttingsections which comprises forming the teeth in the "first sectionrelieved along a truncated cone, forming the ,teeth in the secondsection relieved along a cylinder and forming the teeth in the thirdsection relieved only in the rear portions of said teeth thus leavingthe front portions thereof unrelieved.

5. The process of making worm hobs which comprises milling hob teethparallel to the pitch helix, relieving the tops of the teeth and backingoff the cutting edges along helixes of longer and shorter lead,respectively, than the pitch helix.

6. The process of making worm hobs which comprises milling hob teethparallel to the itc helix, relieving the tops of the teeth. hacking offthe cutting edges alon helixes of longer and shorter lead, respectivethan the pitch helix and trimming said teet cylindriizlallly alonghelixes parallel with said pitch Signed at Detroit, Michigan, this 20thday of August, 1928.

JOHN W. BRUSSEL.

are known. To determine the lead L of the I v flutes G, Fig. 8, we write2R1r 1 tand tan2 d (4) which gives the lead of flutes.

What Iclaim is 1. A- method-of manufacturing worm hobs, the tooth formof which does not change after sharpening, which consists in relievingthe tops of teeth with a uniform relief from the

